Mechanics of Muscle Function

Overview of Skeletal Muscle Function

Skeletal muscles attach to the bony skeleton and are responsible for producing movements. The physical parameters that define how a muscle acts on the skeleton include the arrangement of fascicles within a muscle, principles of lever mechanics, and the position of a muscle as it crosses a joint.

• Fascicle arrangement determines the action and efficiency of a muscle.

• Lever mechanics describe how bones and muscles interact to produce movement.

• Muscle position relative to a joint influences the type of movement produced.

Arrangement of Fascicles in Muscles

Types of Fascicle Arrangement

Fascicles are bundles of muscle fibers arranged in specific patterns, which influence muscle action.

• Circular: Fascicles arranged in concentric rings, typically surrounding external body openings. These muscles are called sphincters (e.g., orbicularis oris, orbicularis oculi).

• Convergent: Broad origin with fascicles converging toward a single tendon (e.g., pectoralis major).

• Parallel: Fascicles run parallel to the long axis of the muscle. Includes straplike (e.g., sartorius) and fusiform (e.g., biceps brachii) muscles.

• Pennate: Fascicles attach obliquely to a tendon. Types include unipennate (one side), bipennate (both sides), and multipennate (multiple sides, e.g., deltoid).

Functional Implications

• Muscles with long, parallel fibers have a greater range of motion.

• Pennate muscles contain more fibers and are more powerful.

Lever Systems: Bone-Muscle Relationships

Principles of Leverage

Movement of skeletal muscles involves leverage, which is described by the following components:

• Lever: A rigid bar (bone) that moves.

• Fulcrum: A fixed point (joint).

• Effort: The applied force (muscle contraction).

• Load: The resistance (body part or object moved).

Mechanical Advantage and Disadvantage

• Mechanical advantage: Effort arm is longer than load arm, allowing a smaller force to move a larger load.

• Mechanical disadvantage: Effort arm is shorter than load arm, requiring more force but increasing speed and range of movement.

Classes of Levers

• First-class lever: Fulcrum is between effort and load (e.g., seesaw, scissors, lifting head off chest).

• Second-class lever: Load is between fulcrum and effort (e.g., wheelbarrow, standing on tip-toe).

• Third-class lever: Effort is between fulcrum and load (e.g., tweezers, flexing forearm by biceps brachii).

Muscle Actions and Interactions

Types of Muscle Roles

Muscles work in groups to produce and control movement.

• Prime mover (agonist): Main muscle responsible for a movement.

• Antagonist: Opposes or reverses the action of the prime mover.

• Synergist: Assists the prime mover by adding force or reducing unwanted movements.

• Fixator: Stabilizes a bone or joint during movement.

Muscle Position and Movement

• Muscles crossing the anterior side of a joint produce flexion.

• Muscles crossing the posterior side produce extension.

• Lateral crossing produces abduction; medial crossing produces adduction.

Organization Scheme Based on Embryonic Development

Muscle Development and Groups

Muscles are organized into developmental and functional groups. They develop from mesodermal structures called myotomes and somitomeres.

• Visceral organ muscles: Smooth and cardiac muscle from splanchnic mesoderm.

• Pharyngeal arch muscles: Skeletal muscles of the pharynx, head, and neck from 4th–7th somitomeres.

• Axial muscles: Muscles of the trunk, neck, and some head muscles from myotomes and somitomeres.

• Limb muscles: Arise from lateral parts of myotomes; extensors and flexors are organized based on their position relative to limb bones.

Muscle Compartments of the Limbs

Compartmental Organization

Dense connective tissue divides limb muscles into compartments, each with similar actions and innervation.

• Muscles in the same compartment act as synergists.

• Opposing compartments contain agonist-antagonist pairs.

• Each compartment is innervated by a single nerve.

Upper Limb Compartments

• Anterior arm: Flexes shoulder/forearm; innervated by musculocutaneous nerve.

• Posterior arm: Extends forearm; innervated by radial nerve.

• Anterior forearm: Flexes wrist/digits; innervated by median/ulnar nerve.

• Posterior forearm: Extends wrist/fingers; innervated by radial nerve.

Thigh and Leg Compartments

• Thigh: Posterior (extends hip/flexes knee, tibial nerve), anterior (flexes hip/extends knee, femoral nerve), medial (adducts thigh, obturator nerve).

• Leg: Posterior (plantar flexes/flexes toes, tibial nerve), anterior (dorsiflexes/extends toes, deep fibular nerve), lateral (plantar flexes/everts foot, superficial fibular nerve).

Naming the Skeletal Muscles

Criteria for Muscle Names

Muscles are named based on several characteristics:

• Location: e.g., brachialis (arm).

• Shape: e.g., deltoid (triangular).

• Size: e.g., gluteus maximus, minimus.

• Direction of fibers: e.g., rectus abdominis, transversus abdominis.

• Attachment points: e.g., brachioradialis.

• Number of origins: e.g., biceps, triceps, quadriceps.

• Action: e.g., flexor, extensor, adductor, abductor.

Muscles of the Head—Facial Expression

Facial Muscles

Muscles of facial expression are thin, variable in shape, and often insert into the skin rather than bones. They are innervated by the facial nerve (cranial nerve VII).

• Examples include frontal belly of epicranius, corrugator supercilii, orbicularis oculi, zygomaticus major, orbicularis oris, mentalis, and platysma.

Muscles of Mastication and Tongue Movement

Mastication Muscles

Four main pairs of muscles are involved in mastication, innervated by the mandibular division of the trigeminal nerve (cranial nerve V).

• Prime movers of jaw closure: masseter and temporalis.

• Side-to-side movement: pterygoid muscles.

• Compression of cheeks: buccinator muscles.

Extrinsic Muscles of the Tongue

These muscles move the tongue in various directions and are innervated by the hypoglossal nerve (cranial nerve XII).

• Include genioglossus, styloglossus, and hyoglossus.

Muscles of the Anterior Neck and Throat—Swallowing

Neck Muscles for Swallowing

The neck is divided into anterior and posterior triangles by the sternocleidomastoid. The anterior triangle contains suprahyoid and infrahyoid muscles, which participate in swallowing. Pharyngeal constrictors squeeze food into the esophagus.

Muscles of the Neck and Vertebral Column

Head and Neck Movement

• Flexion: sternocleidomastoid.

• Lateral flexion: splenius capitis, splenius cervicis.

• Extension: trapezius, splenius.

Trunk Extension

Deep muscles of the back, including the erector spinae group, maintain normal curvatures of the spine and form a column from sacrum to skull.

Deep Muscles of the Thorax—Breathing

Intercostal Muscles

Both external and internal intercostals are involved in inspiration and active expiration. Accessory muscles of ventilation are recruited during forced breathing.

Diaphragm

The diaphragm is the most important muscle of respiration, flattening as it contracts to increase thoracic cavity volume.

Muscles of the Abdominal Wall

Abdominal Wall Structure

The lateral and anterior abdominal wall is formed from three flat muscle sheets: external oblique, internal oblique, and transversus abdominis. The rectus abdominis inserts at the linea alba.

Muscles of the Pelvic Floor and Perineum

Pelvic Diaphragm

The pelvic floor is a sheet of two muscles (levator ani and coccygeus) that support pelvic organs.

Perineum

The urogenital diaphragm is formed from sphincter urethrae and deep transverse perineus.

Superficial Perineal Muscles

Includes ischiocavernosus, bulbospongiosus, and superficial transverse perineus.

Superficial Muscles of the Thorax

Movements of the Scapula

Muscles such as pectoralis major, pectoralis minor, serratus anterior, and subclavius move the scapula.

Posterior Thorax Muscles

Movers of the scapula include trapezius, levator scapulae, rhomboid major, and rhomboid minor.

Muscles Crossing the Shoulder Joint

Major Movers of the Arm

The shoulder joint is highly flexible but unstable. The three largest movers are deltoid, pectoralis major, and latissimus dorsi.

Muscles Crossing the Elbow Joint

Forearm Extensors and Flexors

• Posterior muscles: Extensors (triceps brachii, anconeus).

• Anterior muscles: Flexors (biceps brachii, brachialis, brachioradialis).

Muscles of the Forearm

Movements of the Wrist, Hand, and Fingers

• Tendons are anchored by flexor and extensor retinacula.

• Most forearm muscles originate on the distal humerus.

• Movements include flexion, extension, abduction, and adduction.

Anterior Flexor Compartment

• Includes pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis.

• Innervated by median and ulnar nerves.

Deep Anterior Muscles

• Includes flexor pollicis longus, flexor digitorum profundus, pronator quadratus.

Posterior Extensor Compartment

• Includes brachioradialis, extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris.

• Innervated by the radial nerve.

Deep Posterior Muscles

• Includes supinator, abductor pollicis longus, extensor pollicis brevis/longus, extensor indicis.

Intrinsic Muscles of the Hand

Fine Movement of the Fingers

All intrinsic hand muscles are located in the palm and control precise movements such as adduction, abduction, and opposition.

• Thenar muscles: Ball of thumb (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, adductor pollicis).

• Hypothenar muscles: Abductor digiti minimi, flexor digiti minimi, opponens pollicis.

• Midpalmar muscles: Lumbricals, palmar interossei, dorsal interossei.

Muscles Crossing the Hip and Knee Joints

Thigh and Leg Movements

• Anterior muscles: Flex thigh, extend leg at knee.

• Posterior muscles: Extend thigh, flex leg.

• Adductor muscles: Medial aspect, adduct thigh.

Anterior and Medial Muscles of the Thigh

• Origin on pelvis or vertebral column: iliacus, psoas major, sartorius.

• Medial compartment: adductor longus, adductor brevis, adductor magnus, pectineus, gracilis.

Quadriceps Femoris

• Four heads: rectus femoris, vastus lateralis, vastus medialis, vastus intermedius.

• Common insertion at quadriceps tendon; powerful knee extensors.

Posterior Gluteal Muscles and Lateral Rotators

• Origin on pelvis or sacrum: gluteus maximus, gluteus medius, gluteus minimus.

• Lateral rotators: piriformis, obturator externus/internus, superior/inferior gemellus, quadratus femoris.

Hamstrings

• Biceps femoris, semitendinosus, semimembranosus.

Muscles of the Leg

Compartmental Organization

The fascia lata surrounds leg muscles, divides them into three compartments, and aids venous return.

• Tendons are held in place by extensor, fibular, and flexor retinacula.

• Muscle movement occurs at ankle and intertarsal joints.

Anterior Compartment

• Tibialis anterior, extensor digitorum longus, fibularis tertius, extensor hallucis longus.

Lateral Compartment

• Fibularis longus, fibularis brevis.

Posterior Compartment

• Superficial: triceps surae (gastrocnemius, soleus, plantaris).

• Deep: popliteus, flexor digitorum longus, flexor hallucis longus, tibialis posterior.

Intrinsic Muscles of the Foot

Toe Movement and Foot Support

These muscles help flex, extend, abduct, and adduct the toes, and support the arches of the foot.

• Single muscle on dorsal aspect: extensor digitorum brevis.

• Many muscles on plantar aspect, organized in four layers.

First Layer

• Flexor digitorum brevis, abductor hallucis, abductor digiti minimi.

Second Layer

• Flexor accessorius, lumbricals.

Third Layer

• Flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis.

Fourth Layer

• Plantar and dorsal interossei.

Surface Anatomy

Palpation and Living Anatomy

Palpation is the process of feeling internal structures through the skin, which provides important information about arterial pulses, skeleton, muscles, blood vessels, and the sounds of the heart and lungs. It is also used to determine sites for injections. Additional info: This study guide covers the structure, function, and organization of muscles in the human body, including their development, naming conventions, and compartmentalization. It is suitable for exam preparation in Anatomy & Physiology courses.